Steels which are used for ships, marine structures, and pressure vessels are joined by performing welding so as to form the specified final shapes of the structures. Therefore, it is needless to say that such steels are required to have high strength and excellent toughness for a base metal from the viewpoint of the safety of the structures, and in addition, such steels are required to be excellent in terms of the toughness of a welded joint (weld metal) and a heat-affected zone.
Absorbed energy in a Charpy impact test has mainly been used as an evaluation standard for evaluating the toughness of steel in the past. Nowadays, a Crack Tip Opening Displacement Test (hereinafter, referred to as a CTOD test) is often used in order to increase reliability of the evaluation. In this test, resistance to the occurrence of brittle fracture is evaluated by performing a three-point bend test on a test piece which has been given a fatigue precrack in a region whose toughness is to be evaluated in order to determine the amount of opening (the amount of plastic deformation) of the crack immediately before a fracture occurs.
Since a fatigue precrack is used in a CTOD test, the toughness of a very small region is evaluated. Therefore, a CTOD test may indicate low toughness in the case where a local embrittlement region is present, even if a Charpy impact test indicates good toughness.
A local embrittlement region tends to be formed in a heat-affected zone (hereinafter, also referred to as a HAZ) which is subjected to a complex thermal history due to multilayer weld being performed on, for example, a heavy wall thickness steel, and a bond (the boundary between a weld metal and a base metal) or a region in a bond which is reheated in a temperature range in which a dual phase is formed (a region in which there is an increase in grain diameter in the first cycle of welding, which is reheated in a temperature range in which a ferrite-austenite dual phase is formed in the subsequent welding passes, and which is, hereinafter, referred to as a region reheated in a dual-phase temperature range) becomes a local embrittlement region.
Since a bond is exposed to a high temperature just below the melting point, there is an increase in austenite grain diameter. Therefore, the austenite phase tends to transform into an upper bainite phase having low toughness when cooling is subsequently performed, which results in a decrease in the toughness of a matrix. In addition, since an embrittlement structure such as a Widmannstaetten structure or a martensite-austenite constituent (MA) tends to be formed in a bond, there is a further decrease in toughness.
In order to increase the toughness of a heat-affected zone, for example, a technique in which TiN is finely dispersed in steel in order to suppress an increase in the austenite grain diameter or in order to utilize TiN as a ferrite nucleation site has been put into practice. However, since there is a case where a bond is heated up to a temperature in a range in which TiN is dissolved, the stricter the requirement for low-temperature toughness, the less likely the effects described above are to be realized.
On the other hand, Patent Literature 1 and Patent Literature 2 disclose a technique in which the toughness of a weld zone is increased by adding a combination of a rare earth metal (REM) and Ti and by dispersing fine particles in steel in order to suppress an increase in austenite grain diameter.
In addition, a technique in which a Ti oxide is dispersed, a technique in which the ferrite nucleation capability of BN and oxide dispersion are combined, and a technique in which toughness is increased by further adding Ca and REM in order to control the form of sulfides are proposed in order to increase toughness.
However, these techniques, which are intended for a steel material having comparatively low strength and small alloy content, cannot be applied to a steel material having comparatively high strength and high alloy content, because a HAZ structure including no ferrite phase is formed.
Therefore, as an example of a technique with which a ferrite phase tends to be formed in a heat-affected zone, Patent Literature 3 discloses mainly a technique in which Mn content is increased to 2 mass % or more. However, since Mn tends to be segregated in the central part of a slab in the case of a continuously cast slab, there is an increase in the hardness of a center segregation part not only in a base metal but also in a heat-affected zone, and the region becomes the origin of a fracture, which results in a decrease in the toughness of the base metal and the HAZ.
On the other hand, in a region reheated in a dual-phase temperature range, since carbon is concentrated in a region composed of a reverse-transformed austenite phase formed by performing heating in a temperature range in which a dual phase is formed, a brittle bainite structure including a martensite-austenite constituent is formed, which results in a decrease in toughness. Therefore, techniques in which toughness is increased by decreasing C content and Si content in steel in order to suppress the formation of a martensite-austenite constituent and in which sufficient strength for a base metal is achieved by adding Cu are disclosed (for example, Patent Literature 4 and Patent Literature 5). These are methods in which strength is increased through precipitation strengthening using Cu. Patent Literature 4 uses a method in which a cooling rate after rolling has been performed is controlled to be 0.1° C./s or less so that Cu particles are precipitated in the cooling process. There is a problem to be solved regarding manufacture stability in the case of the method according to Patent Literature 4. In addition, in the case of Patent Literature 5, a decrease in toughness due to an increase in the grain diameter of AlN and the negative effect of solid solute N is suppressed by controlling a N/Al ratio to be 0.3 to 3.0. However, the negative effect of solid solute N can be suppressed more easily using Ti.